Use distutils from Python standard library

setuptools 60.0 switched from standard library distutils to its own
local distutils by default [1]. setuptools' own distutils version
doesn't support building with 2to3 translation anymore [2].

However, the current pycryptodomex version (3.6) relies on distutils
2to3 translation support for correct Python 3 builds. Building with the
setuptools distutils version results in run-time errors such as

  . . .
    File "/opt/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/cms-1.5.dev0-py3.8.egg/cmscommon/crypto.py", line 31, in <module>
      from Cryptodome.Cipher import AES
    File "/opt/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/Cryptodome/Cipher/__init__.py", line 31, in <module>
      from Cryptodome.Cipher._mode_ctr import _create_ctr_cipher
    File "/opt/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/Cryptodome/Cipher/_mode_ctr.py", line 37, in <module>
      from Cryptodome.Util.number import long_to_bytes
    File "/opt/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/Cryptodome/Util/number.py", line 399
      s = pack('>I', n & 0xffffffffL) + s
                                   ^
  SyntaxError: invalid syntax

We set the SETUPTOOLS_USE_DISTUTILS environment variable to continue
using the system distutils version with 2to3 translation support for
now. The variable can be removed later when we update to a newer
pycryptodomex version.

In the documentation, we also pass the environment variable to the
"setup.py install" command for consistency.

[1] https://github.com/pypa/setuptools/commit/f7a55da560b35c5edff2795a5c9ee5ac3985668f
[2] https://github.com/pypa/setuptools/commit/d989cdb36f50785d23b07939ba8b4fc2b68cf02a

.github/workflows/main.yml

@@ -51,6 +51,9 @@ jobs:
 
       - name: Install requirements
         run: |
+          # Use distutils from the standard library for 2to3 translation
+          export SETUPTOOLS_USE_DISTUTILS="stdlib"
+
           pip install -U pip setuptools wheel
           pip install -r requirements.txt
           pip install -r dev-requirements.txt

docs/Installation.rst

@@ -145,13 +145,15 @@ Assuming you have ``pip`` installed, you can do this:
 
 .. sourcecode:: bash
 
-    sudo pip3 install -r requirements.txt
-    sudo python3 setup.py install
+    export SETUPTOOLS_USE_DISTUTILS="stdlib"
+    sudo --preserve-env=SETUPTOOLS_USE_DISTUTILS pip3 install -r requirements.txt
+    sudo --preserve-env=SETUPTOOLS_USE_DISTUTILS python3 setup.py install
 
 This command installs python dependencies globally. Note that on some distros, like Arch Linux, this might interfere with the system package manager. If you want to perform the installation in your home folder instead, then you can do this instead:
 
 .. sourcecode:: bash
 
+    export SETUPTOOLS_USE_DISTUTILS="stdlib"
     pip3 install --user -r requirements.txt
     python3 setup.py install --user
 
@@ -176,6 +178,7 @@ After the activation, the ``pip`` command will *always* be available (even if it
 
 .. sourcecode:: bash
 
+    export SETUPTOOLS_USE_DISTUTILS="stdlib"
     pip3 install -r requirements.txt
     python3 setup.py install